Radio frequency (RF) devices transmit an information signal from one point to another by moving the information signal to a higher frequency range that is more suitable for transmission over the medium being used. This process is known as upconversion. As used herein, “radio frequency signal” means an electrical signal conveying useful information and having a frequency from about 3 kilohertz (kHz) to thousands of gigahertz (GHz), regardless of the medium through which such signal is conveyed. Thus an RF signal may be transmitted through air, free space, coaxial cable, fiber optic cable, etc. An RF transmitter mixes the desired signal, known as the baseband signal, with an RF carrier frequency for transmission over the selected medium. An RF receiver then mixes the signal with the carrier frequency to restore the signal to its original frequency.
A superheterodyne receiver is a receiver that mixes the desired data-carrying signal with the output of local oscillator to produce an output at a fixed intermediate frequency (IF). The fixed IF signal can then be conveniently filtered and converted back down to baseband for further processing. Superheterodyne receivers are useful in a wide variety of applications in which the desired channel can occur within a wide band of frequencies, such as AM and FM radio, satellite radio, etc.
To reduce the cost of a superheterodyne radio receiver, it is useful to combine as many circuit elements as possible into a single integrated circuit (IC). While contemporary complementary metal-oxide-semiconductor (CMOS) technology can operate fast enough to process RF and IF signals, certain other functions cannot be easily integrated into CMOS processes. For example, inductors and capacitors may be fabricated on low-cost CMOS ICs to form LC filters, but LC filters do not provide a very good frequency response to filter the selected IF. Another known type of filter that provides better channel selectivity than an LC filter is known as a surface acoustic wave (SAW) filter. A SAW filter is a piezoelectric device that converts an electrical signal to a mechanical vibration signal and then back to an electrical signal. Filtering is achieved through the interaction of signal transducers in the conversion process.
However a filter of this type is typically constructed using zinc oxide (ZnO2), a material that is as yet incompatible with integration on a CMOS circuit utilizing a silicon substrate. Thus a low-cost CMOS receiver may benefit from the use of an external SAW filter and have input terminals connected to the output terminals of the SAW filter. Existing SAW filters can be modeled as voltage sources having a characteristic source impedance, for example a source resistance in parallel with a source capacitance. In order for the filter to operate with the desired bandpass properties, it is necessary to terminate the source resistance of the SAW filter with a matching resistance before the signal can be mixed from IF to baseband. However resistors integrated into CMOS processes are characteristically noisy. Thus better circuits for terminating signals from, for example, external SAW filters would be desirable.